Navien NHB-110 Boiler For Hot Water Hydronic BaseBoard Heating Howling

Could be the gas pipe. It appears to be CSST feeding that boiler but I can't be sure. I have experienced this with CSST gas pipe making the howling sound when the burner is at maximum input. The fix is to go the the next size larger CSST pipe, or run the proper size gas pipe using iron / steel pipe and fittings.

How long is the black covered steel gas pipe? If it is more than about 6 feet, then it is probably too small, looks like 3/4" in the photos. You need at least 1" for any distance and you may need 1-1/4" if the run is over 30 feet.

I listened to the video again and that does not sound like the "Fog Horn" noise that I have experienced that results from undersized CSST.  So now I’m up to 92% sure it’s not the gas pipe.

I did notice that you have 5 circulator pumps and I am thinking there might be too much water flow thru the common pipe.  Just using a rule of thumb for pipe sizes and the fact that the Taco 007 pumps can move over 20 GPM in certain conditions, let's look at what might be going on in that neat boiler piping.   

Using this rule of thumb for pipe size and recommended GPM needed to reduce noise, we can add up all the pump’s possible flow rates to see what the total might be.  Remember this is not the actual GPM flow rate, that can only be measured with some calculations.  I am just pointing out a possibility.  That’s all.  

Four of your five pumps are connected to ¾” copper that can move up to 4 GPM under ideal operating conditions.  That can be a total of 16 GPM for those 4 pumps.  The fifth pump is connected to a larger pipe. At least 1-¼” but it may be 1-½” copper pipe.  If it is 1-¼” pipe that has a capacity of 14 GPM for a total of 30 GPM thru the shared piping.   Looking at the near boiler piping, the primary loop appears to be 1-¼” copper close to the boiler, then increases to 1-½” copper when it turns up to the ceiling to then go over to the circulator pump manifold.   Since you have the possibility of moving 30 GPM thru the shared piping and you have only 1-¼” of shared piping near the boiler, I might look at repiping the section of pipe indicated in this photo in Blue highlight from 1-¼” copper to 1-½” copper

A test to prove this “too much water flow” theory would be to adjust all 5 circulator zones to a lesser flow by closing each zone about 80 to 85%** with the ball valve on the discharge side of the pump.  This will reduce the GPM in each zone and still provide sufficient heat for the building.  When the reduced flow makes the noise dissipate, you will know that the noise is from the 1-¼” shared pipe near the boiler.   

** adjusting flow with a ball valve does not do very much with the first 70% of the valve throw.  You only actually start getting a measurable restriction after that amount of closure.  

Your flue & fresh air pipe looks to me to be 2" PVC pipe. For that length, I think it should be 3" pipe. A pic of the termination outside would be helpful. When the howling is going, I would have someone outside placing their palm over the exhaust outlet and see what the sound is at the boiler. That may tell you if it is a exhaust problem.

I just hate it when I see cast iron mixed with copper. A retro fit from steam to hot water, I can see it. But, that's me, whata I know.

Your piping of the boiler violates some hydronic principles. It is more convoluted than necessary. The inlet to the boiler pump should be at least 10 diameters of straight pipe and the outlet should be 5 diameters of straight pipe before any change of direction. Running pipe like it was a domestic water sys with higher pressures let one get away with simple errors. But, a hydronic sys with low pressures can create problems if one doesn't minimize pressure losses in the piping arrangement.

I know that you just want to get rid of the howling. I would look at the exhaust and air intake sys.

A mechanic's stethoscope that you can get at an auto parts store or Harbor Freight may help you pinpoint the origin of the sound.

Take a close look at the pipes in the photo. I believe that the installer used a reducing 90° elbow here. Check the outer diameter of the vertical pipe and compare it to the horizontal pipe.

Is it possible that you have 2” copper between the near boiler piping and the risers that go to the circulator pump area?

Since it does not cost anything to adjust a valve, it can't hurt to try the adjustment. I also agree with @hot_rod that you are probably not pumping the maximum in each zone, but since it is free to adjust, you can certainly start at about 70% then75% then 80% but I don't believe that I would go much past 85% or you may reduce the actual heating capacity of the zone(s)

OMG. I just looked at the optical illusion created by the 1/2" water feed pipe next to the vertical pipe when I put the red arrows on that photo

Also combustion noise is a possibility. If you look at the manual there should be a maximum vent length for different vent diameters. Make sure you are not exceeding the total equivalent length (EL) of the vent pipe. Sometimes each 90° elbow will add 5 to 10 feet of EL of strait pipe. so 20 feeet of pipe with 4 elbows might actually be 60 feet total equivalent length.

If the noise varies with the load the boiler is under it isn't a water flow issue unless you have variable speed circulators.

My guess is combustion related/venting.

One thing you could try is limit the boiler input and see if the noise diminishes when all zones are calling.

Try partially closing the supply valves maybe 1/4 and see if the sound changes.

The zones

Removing the air intake pipe will tell you if the inlet pipe is too long for the diameter you are using. As long as there is enough combustion air in the boiler room by leaving doors open and perhaps a window open to allow for outside air to get to the boiler location. When the howling stops as soon as you disconnect the intake, you have found your problem. The fix is a larger diameter vent pipe

Lets say it does not completely stop the howling, then cut the vent pipe just before the first elbow (with room to add a coupling) and see it that makes the problem stop. If it does, then the fix is a larger diameter exhaust pipe. Put the pipe back together with a coupling to heat your home then order to larger diameter intake and exhaust piping to get the job done.

Good luck with getting rid of your werewolf.

I would say that vent is suspect to say the least.. If we ignore that the manufacturer almost definitely doesn't approve that much venting being outside, if a little wind hits the exhaust it will just swirl around under that overhang and get sucked right back in the intake. Not to mention the mini split being a little bit in the line of fire as well.

@ScagDriver1037, I also use this sound detector. I originally bought it for my car. but works great for the boiler, too.

https://www.amazon.com/ZKTOOL-Electronic-Stethoscope-Diagnostic-Headphones/dp/B09QLXV61Q?crid=3G8LA7T295NMF&dib=eyJ2IjoiMSJ9.i0Ui9A0yLttv4Aj27sXvv-EOPDA_nFh0SOBgRxDppFr6U5r7fkhXE-hLivyKTZHAb95iOHglQ3EdifZRNGC7fv4CRrB1MIhcf-0_GghXQbm92ny24ZxE5OiutOGHzoGj9eoq3vNB-5161KFYRyt0MlxYnm4eshfSthFvQOVyXIBMysjWjzQrPG4rL0UkvEq7QqdXktpMQ-k0Lp85lFpV_VUD_5yImHRqyuGM2uxalcA.ZjiLGLyKayBWXKGkkPOhuji46eWD43r5SPuAnCJMac4&dib_tag=se&keywords=automotive%2Bsound%2Bdetection&qid=1770063929&sprefix=automotive%2Bsound%2Bdetection%2Caps%2C208&sr=8-3&th=1

@ScagDriver1037 , I never use this, ever. Did I say ever? I do not trust this venting. I have seen it on the windward side of the building, which can be a big problem with gas reversion.

I do a variation of this.

@ScagDriver1037 , sorry I din't see your outside termination pic. Looks ok. I like the way you brought the exhaust outside of the decking. Lot of moisture in the exhaust. I have done that, too, but in my area the winters are really cold, so I insulated the pvc to prevent freezing of moisture in the exposed pipe.

It should run back to the boiler yes. Sometimes I have observed where the pipe sort of creates a "belly" sometimes its just improper hanger height or not enough support. When the condensate pools a little in a spot it can cause all sorts of issues.